Human Impact On The Nitrogen Cycle - History of Anthropogenic Nitrogen Inputs

History of Anthropogenic Nitrogen Inputs

Approximately 78% of earth's atmosphere is N gas (N₂), which is an inert compound and biologically unavailable to most organisms. In order to be utilized in most biological processes, N₂ must be converted to reactive N (Nr), which includes inorganic reduced forms (NH₃ and NH₄+), inorganic oxidized forms (NO, NO₂, HNO₃, N₂O, and NO₃-), and organic compounds (urea, amines, and proteins). N₂ has a strong triple bond, and so a significant amount of energy (226 kcal mol-1) is required to convert N₂ to Nr. Prior to industrial processes, the only sources of such energy were solar radiation and electrical discharges. Utilizing a large amount of metabolic energy and the enzyme nitrogenase, some bacteria and cyanobacteria convert atmospheric N₂ to NH₃, a process known as biological nitrogen fixation (BNF). The anthropogenic analogue to BNF is the Haber-Bosch process, in which fossil fuel H₂ is reacted with atmospheric N₂ at high temperatures and pressures to produce NH₃. Lastly, N₂ is converted to NO by energy from lightning, which is negligible in current temperate ecosystems, or by fossil fuel combustion.

Until 1850, natural BNF, cultivation-induced BNF (e.g., planting of leguminous crops), and incorporated organic matter were the only sources of N for agricultural production. Near the turn of the century, Nr from guano and sodium nitrate deposits was harvested and exported from the arid Pacific islands and South American deserts. By the late 1920s, early industrial processes, albeit inefficient, were commonly used to produce NH₃. Due to the efforts of Fritz Haber and Carl Bosch, the Haber-Bosch process became the largest source of nitrogenous fertilizer after the 1950s, and replaced BNF as the dominant source of NH₃ production. From 1890 to 1990, anthropogenically created Nr increased almost ninefold. During this time, global population more than tripled, partly due to increased food production.

Since the industrial revolution, an additional source of anthropogenic N input has been fossil fuel combustion, which is used to generate energy (e.g., to power automobiles). During combustion of fossil fuels, high temperatures and pressures provide energy to produce NO from N₂ oxidation. Additionally, when fossil fuel is extracted and burned, fossil N may become reactive (i.e., NO_x emissions). During the 1970s, scientists began to recognize that N inputs were accumulating in the environment and affecting ecosystem functioning.